An extrusion die is used to extrude molten thermoplastic into a film or sheet. A prior type of extrusion die includes a die cavity having a coathanger-shaped manifold at an upstream portion of the die, a triangular preland portion downstream of the manifold, a melt well (otherwise referred to as a secondary relief or secondary manifold) downstream of the preland portion and a die exit adjacent the melt well. In this type of die, a back line (or upstream edge) of the preland portion includes linear edges forming a taper which converges toward a die entrance. The preland portion provides resistance to flow which varies over the width of the die so that the flow of thermoplastic is uniformly transversely spread out.
It has been found that, owing to the rheological characteristics of thermoplastics, the use of a preland portion having a linearly tapered upstream or downstream edge results in less than ideal thermoplastic flow through the die. In particular, in the case where a monolayer flow is passed through the die, the extrudate tends to flow in a fashion which produces an M-shaped or W-shaped flow distribution profile. In the past, this problem was obviated for a given extruder flow rate by adjusting the lip gap at the die exit so that the flow nonuniformity was corrected. Because extruder flow rates in the past were typically maintained substantially constant from job to job, this adjustment did not have to be frequently repeated in order to maintain high quality output. However, more recent attempts at reducing costs by reducing inventories have led to the need to produce short runs of product, thereby necessitating the use of differing extruder flow rates. This undesirably creates the need to frequently readjust the lip gap at the die exit.
In the case where two or more thermoplastic flows are to be coextruded in separate layers, the use of a conventional die as described above results in the production of a layer boundary (or layer boundaries) having an M or W shape. Such an interface deformation (or encapsulation) results not only from the linear preland configuration but also from the manifold design. This deformation cannot be eliminated by adjusting the die lip gap. In addition to the foregoing effect, the overall film will have the M or W shape noted above in connection with monolayer applications.
Appel U.S. Pat. No. 4,043,739 discloses an extrusion die of the coathanger type wherein a back line of a preland portion is nonlinearly tapered, i.e., the edges defining the taper are curved. Also, in this die the depth of a manifold portion upstream from the preland portion (i.e., the distance between assembled die halves in the manifold portion) decreases in a nonlinear fashion from a centerline of the die to outside ends thereof. A similar die is sold by assignee of the instant application and is referred to as the "Multiflow III" die. In at least the case of this latter die, the production of M or W shapes in the cross-section of the produced monolayer or multilayer sheet is substantially reduced or eliminated.
Prior art coathanger-type extrusion dies, such as those described above, have encountered a problem known as differential clam-shelling which causes an extruded sheet of thermoplastic material to be thicker at the center than at the ends, which is undesirable. This effect is caused by the pressure of the molten thermoplastic material acting against moment arms of differing lengths resulting from the anchoring of die halves at points unequally spaced from the die exit.
In an extrusion apparatus referred to as the "Epoch" die manufactured and sold by the Cloeren Company of Orange, Tex., an inverted preland portion separates a manifold from a melt well and a die exit. The inverted preland portion includes an upstream edge or back line which extends as a straight line from one end of the die to the other perpendicular to the centerline of the die and further includes a downstream edge which is nonlinearly tapered toward the die exit. A claim is made by the manufacturer of this die that, because of the straight leading edge, the back line of the manifold may similarly be configured to extend straight across the die width perpendicular to the centerline of the die, thereby resulting in the ability to locate body bolts holding the die halves together at points equally spaced from the die exit. However, it is believed that the Epoch die does not truly have a straight or nontapered manifold back line owing to the need to have the manifold diminish in length toward the die ends, and hence there is no way to locate the body bolts at equally spaced points from the die exit without causing some bolts to be spaced farther from the manifold back line than others, thus substantially increasing the risk of developing a leak between the die halves at the manifold back line.
Further, because the downstream edge of the preland portion of the Epoch die is tapered, the melt well flow path increases in length from the center of the die to the outside portions of the die, thereby introducing undesirable disturbances in the thermoplastic melt flow.